1. Field of the Invention
The present invention relates to an electric vehicle such as an electric automobile having an electric motor for producing a drive force (output torque) to be transmitted to drive road wheels to propel the electric vehicle.
2. Description of the Prior Art
Electric automobiles have a battery for storing electric energy that is used to energize an electric motor to rotate drive road wheels. In order to increase the milage per one charging cycle for the battery, it is preferable to operate the electric motor in a high-energy efficiency range, i.e., at a high ratio of mechanical output energy produced by the electric motor to electric input energy applied to the electric motor.
One electric automobile with increased energy efficiency is disclosed in Japanese laid-open patent publication No. 62-107607, for example.
The disclosed electric automobile is designed such that the output torque and rotational speed of the electric motor are increased along a high-efficiency curve where the energy efficiency of the electric motor is highest until a target automobile speed is reached when the automobile is propelled in response to the depression of an accelerator pedal. The electric motor is thus controlled to operate in a high range of energy efficiencies.
However, since the wheel drive force of the electric vehicle, i.e., the output torque at the drive road wheels, and the vehicle speed of the electric vehicle are determined according to the output torque and rotational speed of the electric motor, the wheel drive force may often become insufficient when the electric motor is energized along the high-efficiency curve. When the wheel drive force becomes insufficient, therefore, the output torque of the electric motor is increased such that the electric motor is energized in a range far from the high-efficiency curve, i.e., in a range of relatively low energy efficiencies for the electric motor.
The electric motor is actually energized less frequently along the high-efficiency curve under actual running conditions. Inasmuch as the electric motor is not often energized in a high range of energy efficiencies, the energy efficiency of the electric motor is not sufficiently high in actual operation.